This invention relates generally to a controller of the type suitable for activating a switch device of an electronic type, such as may be used in a combustible fuel burner. More specifically, the invention relates to a microprocessor-based controller apparatus for executing a linear program in synchrony with a recurring inverted signal coupled to the reset terminal of the microprocessor. The preferred embodiments of the invention employing the microprocessor-based controller will be described with reference to combustible fuel burner systems, however the invention is not so limited. The invention is in fact applicable in any application where a high reliability microprocessor-based control circuit is desired.
A variety of gas ignition controls are known and generally include an apparatus constructed of discrete components for effecting a single, predetermined control sequence tailored to a specific application. Such gas ignition systems typically include a relay which is energized in response to a thermostatic demand for heat and for actuating a pilot gas valve, spark generating circuit for igniting the pilot gas either coincidently with the energization of the pilot valve or within a predetermined time thereafter, a flame-sensing probe for detecting the presence of a pilot flame and an output relay section for energizing a main fuel valve upon a detection of the pilot flame. Refinements of this fundamental system may include a pair of electromagnetic relays which are required to be operated in a particular sequence upon each reception of a thermostatic demand for heat. The sequential operation of these relays is effective in verifying the integrity of certain circuit components. Examples of such gas ignition systems are shown and described in U.S. Pat. Nos. 4,077,762 and 4,178,149. While these gas ignition systems have heretofore been satisfactory, they fail to appreciate the manner in which a microprocessor may be utilized to continually monitor and verify the integrity of certain circuit components.
An example of a microprocessor-based controller is disclosed in U.S. Pat. Nos. 4,581,697 and 4,931,975, assigned to the assignee of the present invention. These microprocessor-based apparatus make use of the capabilities of the microprocessor to monitor various variables within the entire system to created a moderately high rate of reliability. However, in a combustion fuel burner and other applications requiring a very high degree of reliability, it is necessary to predict and/or limit the possible failure modes which may be cause by a malfunction in the timing or internal working of the microprocessor itself. Conventional microprocessor-based systems, driven by software, can theoretically have an infinite number of failure modes, some of which may be due to a failure within the physical structure of the microprocessor itself. These microprocessor controllers are typically operated on the positive portion of the input power supply, during which time the output circuits of the system are also able to be operated. Therefore, a failure in the microprocessor could result in the operation of the output circuits in the wrong sequence or at inopportune times. Accordingly, a microprocessor-based control system offering a high degree of reliability which takes into account possible failures within the microprocessor would be a distinct advance in the art.